Plastic and reconstructive surgery often requires the use of graft materials for the replacement or augmentation of tissues. Materials used for this purpose heretofore have been of biologic or synthetic origin. Biologic materials of both autologous and homologous origin have been tried extensively. Both types of biologic material have been subject to unpredictable resorption, requiring the patient to undergo additional corrective surgery. The use of homologous implant materials, for example, collagen or bone, can also result in an adverse immunologic reaction that can lead to graft rejection and extrusion of the implant material. While such adverse reactions do not occur with autologous implants, the use of autologous material involves additional surgical time and trauma for their removal.
Synthetic materials previously used for implantation have generally been polymeric, for example, silicone and polytetrafluoroethylene (hereinafter PTFE). Non-porous materials do not allow tissue ingrowth and as a consequence are known to migrate from the implant location. Preferred synthetic materials have a porous structure that promotes tissue ingrowth and stabilization of the implanted material.
Proplast.RTM. (Vitek, Inc. Houston, Tex.), a carvable porous composite implant material comprising PTFE fibers, powdered PTFE resin and carbon or aluminum oxide, has been used previously as an implantable material for reconstructive surgery. This material and its methods of manufacture are described in U.S. Pat. Nos. 3,992,725 and 4,129,470. The manufacturing procedure involves blending the above listed materials with a soluble filler, filtering the blend to produce a cake, pressing and heating the cake, drying the cake, sintering the cake, and finally leaching out the filler material and again drying the resulting porous composite. The finished material was claimed to be carvable and to allow tissue ingrowth. However, the use of carbon or aluminum oxide in this material increases its tissue reactivity, potentially resulting in undesirable complications such as encapsulation by fibrous tissue, erosion of overlying tissues and extrusion. Finally, the carbon impregnated material is often visible through the skin when implanted subcutaneously in light-skinned patients.
PTFE without other added materials such as carbon has a long history of use as an implantable material because it is one of the least reactive materials known. In porous form it can allow tissue ingrowth. Porous PTFE has been available for some time in a form known as expanded PTFE. The manufacture of this material is described in U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390. Expanded PTFE has a microstructure characterized by nodes interconnected by fibrils. This material has a history of use in such implant applications as vascular grafts, sutures and structural soft tissue repair such as hernia repair. The porosity and microstructure of expanded PTFE can be varied to produce different permeability characteristics for use in a variety of applications.
U.S. Pat. No. 5,098,779 by the present inventors describes a carvable, implantable porous PTFE material wherein the PTFE is made carvable by a coating or impregnation of a biodegradable stiffening agent. While this is an effective means of providing carvability in porous PTFE, the coating can interfere with the porosity and tissue ingrowth characteristics of the surface of the porous PTFE until enough of the stiffening agent has been absorbed by the surrounding tissue.